1. Field of the Invention
The present invention relates to a plasma processing apparatus and a plasma processing method with which a workpiece to be processed is processed with plasma generated by energizing a gas to plasma with the power of microwaves. More specifically, the present invention relates to a plasma processing apparatus and a plasma processing method, in which the length of a waveguide is optimized by using a desired wavelength-varying substance.
2. Description of the Related Art
There is a technology known in the related art whereby a waveguide 90 is branched at points A1˜A3 (π branches in the example presented in FIG. 7), a plurality of slots 92 are formed over equal intervals under a plurality of branched waveguides 91a˜91f and a dielectric member 93 is disposed under the slots 92 so as to allow the power of microwaves to be supplied into a processing chamber of a plasma processing apparatus, as shown in FIG. 7. Through this power supply method, microwaves emitted from a microwave generator 94 are propagated through the waveguide 90 via a tuner 95, are branched into the individual waveguides 91 so as to travel through the slots at each waveguide 91 starting at, for instance, the slot 92a near the entrance and then through the subsequent slots in sequence ending at the slot 92e, are transmitted through the dielectric member 93 underneath the individual slots, and enter the processing chamber. A gas supplied into the processing chamber is raised to plasma with the electric field energy of the microwaves having entered the processing chamber as described above, and a workpiece is processed with the plasma thus generated.
As illustrated in the upper side of FIG. 8 in a sectional view of the plasma processing apparatus taken through II-II in FIG. 7, each time the microwaves, being supplied by adopting the power supply method described above, travel through one of the slots at the waveguide 91a, starting at the slot 92a closest to the entrance and then in the order of, 92b→92c→92d→92e, the amplitude of standing waves T, which are composite waves constituted with the microwaves advancing through the waveguide 91a and the microwaves reflected from an end surface C of the waveguide, becomes smaller. This means that the level of electrical field energy of the microwaves having traveled through the slots 92 and having entered the processing chamber, too, is higher in a space Ua inside the processing chamber under the slot 92a closest to the entrance to the waveguide and is lower over a space Ue in the processing chamber under the slot 92e closer to the end of the waveguide as indicated by an electrical field energy curve Emax in the lower side of FIG. 8. As a result, the plasma density is higher at positions closer to the entrance of each waveguide and is lower at positions closer to the end of the waveguide. The workpiece cannot be processed accurately with plasma generated in a non-uniform manner along the length of the waveguide as described above.
Uniformity in the electrical field energy of microwaves entering the processing chamber through the plurality of slots 92 may be achieved by adopting the following measures. Namely, the slot 92e in each waveguide, closest to the end surface C, may be set at such a position that its distance to the end surface C in a physical characteristic length is ¼ of the guide wavelength λg of the microwaves, and the slots 92a˜92d may be each set at such a position that the closer the subject slot is to the entrance of the waveguide 91, the physical characteristic length between the subject slot and the adjacent slot deviates from ½ of the guide wavelength λg to a greater extent.
Thus, at the slot 92e, the position Pe of the peak of the standing wave is aligned with the central position Se of the slot 92e (W 1=0), which results in the level of the electrical field energy of microwaves supplied through the slot 92e to the position Ue in the processing chamber becoming equal to the maximum energy Emax for the position. As for the slots 92d through slot 92a, the extent of positional misalignment between the center (Sa−Sd) of the slot and the standing wave peak (Pa−Pd) is greater at a slot closer to the entrance of the waveguide 91 (W 5>W 4>W 3>W 2>W 1). A greater extent of positional misalignment results in the microwave electrical field energy supplied to a specific position among positions Ud, Uc, Ub and Ua in the processing chamber through the corresponding slot 92 being lower than the maximum energy for the position by a greater degree. Consequently, substantial uniformity is achieved in the electrical field energy of microwaves entering via the slots 92a through 92e as indicated by the line E in the electrical field energy of FIG. 8.